Someone needs to come out with a driverless MP3 player similar to what iRiver used to have, with decent (40gb+ hard disk space) capacities. I know its possible. My generic Napster 512MB player uses no drivers, and has two modes, MTP (media transfer protocol, which is transactional), and the regular old USB flash drive volume.
I wish I knew more about embedded programming... grab a copy of an embedded OS , a hard disk, decent small ARM based motherboard with solid DACs, and make a prototype of what I want.
: OS of choice depends on features. I'm pretty sure that AAC compatibility and the GPL v3 don't go together legally, so most likely it will end up being Windows CE.